"You can set a personal function in the linked shooting flash so that it will fire if that is what you are trying to do. It is Personal Function 07. Please follow these steps:
1. Press the button.
2. Press the same button again, which is now the button.
3. Select [P.Fn-07].
4. Press the button.
5. Select [1: ON (Enabled)]."

I am a little embarrassed that this was in the manual. Probably should have kept reading.